This invention relates to the art of pneumatic control equipment, and more particularly relates to a pneumatic device for detecting a phenomenon at one location and producing an output signal at another location indicative of the phenomenon so detected.
Such remote sensing apparatus is used in numerous applications in industrial machinery and the like. For example, in complex automatic machinery having a multitude of reciprocating slides, the presence of a particular slide at the end of its stroke must be detected at a location adjacent to that slide, while the output signal produced in response to such presence must be produced at a central control panel several feet away from that slide.
In this disclosure, the term "signal" is used in its broadest sense and is not limited to active, energetic outputs. Rather, the term "signal" should be understood as also including any status of a mechanical, pneumatic, hydraulic, fluidic or electrical element which can be detected by a further device or which can be detected manually. Thus, for example, the closed status of an electrical switch would constitute a signal, and the open status of the same switch would constitute another, different signal.
Although various electrical and hydraulic arrangements can be utilized for remote sensing, considerations of cost, simplicity of operation, ease of maintainence and safety often favor the use of a pneumatic remote sensing system. Any such system should possess the following desirable characteristics. First, it should produce the appropriate signal immediately upon the occurrence of the phenomenon to be sensed, with the least possible delay. Second, it should requite only a single pneumatic conduit extending between the location where the phenomenon is to be sensed and the location where the output signal is to be produced, so that the system does not unnecessarily encumber the equipment on which it is mounted. Third, it should not require any careful adjustment during installation or use, and should be capable of accomodating substantial variations among installations in the distance between the sensing location and the location where the output signal is to be produced. Fourth, it should be reliable; it should always produce the output signal representative of a phenomenon which has occurred, and it should never produce a false output signal which is representative of a phenomenon which has not occurred.
One remote sensing system is described in U.S. Pat. No. 4,118,612, issued Oct. 3, 1978. In this system, the upstream end of a tubular conduit is connected to a source of compressed air by way of an adjustable throttling valve, and a normally-open pressure sensitive switch is connected to the conduit adjacent to its upstream end. The downstream end of the conduit can be occluded by the operator's finger. Thus, while the downstream end of the conduit is not occluded, air will flow through the throttle valve and the conduit and out through the downstream end of the conduit through the atmosphere. During such operation, the pressure in the conduit at the pressure-sensitive switch will depend on the supply pressure, the throttle valve setting and the flow resistance of the conduit itself. All of these factors must be chosen and maintained so that this pressure is less than the so-called trip pressure required to close the pressure sensitive switch. When the downstream end of the conduit is occluded, continued flow of air into the conduit through the throttle valve causes the pressure within the conduit to rise gradually until it reaches the supply pressure. At some time during this process, the pressure in the conduit reaches the trip pressure and the switch closes, thus producing an output signal indicating presence of the operator's finger. The delay between occlusion of the conduit and closure of the switch is roughly proportional to the difference between the trip pressure and the pressure at the switch during operation while the conduit is not occluded.
While this system is capable of satisfying some of the aforementioned requirements, it cannot satisfy all of them simultaneously. If the supply pressure, the throttle valve setting and the flow resistance of the conduit are chosen and balanced so that the aforementioned pressure difference is small, then the switch will close promptly after the downstream end of the conduit is occluded. However, any small change in the supply pressure or in the flow resistance of the conduit will raise the pressure at the switch above the trip pressure, and will thus cause a false trip to occur while the downstream end of the conduit is not occluded. On the other hand, if the difference is great, the system will be relatively safe from accidental trips, but the delay between occlusion of the conduit and closure of the switch will be substantial.